The alternative sigma factor σE controls antioxidant defences required for Salmonella virulence and stationary-phase survival

Abstract

Bacteria must contend with conditions of nutrient limitation in all natural environments. Complex programmes of gene expression, controlled in part by the alternative sigma factors σS (σ38, RpoS) and σH (σ32, RpoH), allow a number of bacterial species to survive conditions of partial or complete starvation. We show here that the alternative sigma factor σE (σ24, RpoE) also facilitates the survival of Salmonella typhimurium under conditions of nutrient deprivation. Expression of the σE regulon is strongly induced upon entry of Salmonella into stationary phase. A Salmonella mutant lacking σE has reduced survival during stationary phase as well as increased susceptibility to oxidative stress. A Salmonella strain lacking both σE and σS is non-viable after just 24 h in stationary phase, but survival of these mutants is completely preserved under anaerobic stationary-phase conditions, suggesting that oxidative injury is one of the major mechanisms of reduced microbial viability during periods of nutrient deprivation. Moreover, the attenuated virulence of σE-deficient Salmonella for mice can be largely restored by genetic abrogation of the host phagocyte respiratory burst, suggesting that the σE regulon plays an important antioxidant role during Salmonella infection of mammalian hosts.